Initial tests of a new pseudo-single-bunch (PSB) operational mode at the ALS have shown promising results—PSB would vastly expand the facility’s capacity to carry out dynamics and time-of-flight experiments with a major reduction in sample damage. In PSB operation, a single electron bunch is displaced transversely from the other electron bunches circulating in the storage ring. Experiments that require light emitted only from a single bunch can stop the light emitted from the other bunches using a collimator. Other beamlines would only see a small reduction in flux due to the displaced bunch. As a result, PSB complements the scheduling of multibunch and timing experiments—currently, the ALS only offers these capabilities for four weeks out of its yearly operating schedule. With PSB operation, the ability to conduct time-of-flight experiments could be available year-round.

The novel “kick and cancel” (KAC) scheme that ALS developed as part of its new pseudo-single-bunch (PSB) operational mode is key to giving timing experimenters the flexibility they need. In PSB mode, neither the kick angle nor kick polarity can be changed quickly. However, the option of a variable kick frequency exists. For instance, the kicker can be adjusted to kick every turn, every nth turn, or even have an uneven kick pattern such as kicking once, then kicking two turns later, waiting 1 ms and repeating. This turns out to be a very important feature for some beam users who would like to see an adjustable pulse repetition rate from several kHz to a few hundreds of kHz.

The arrival time of the PSB pulses can be also easily controlled and synchronized by users with their timing, which is another important feature for timing experiments, particularly for the laser-pump, x-ray probe experiment. With a relatively simple, inexpensive pulsed kicker magnet that requires only half a meter of a single straight section in the storage ring, it is possible to achieve both single-bunch and multibunch operations at the same time.

The top image shows the ALS ring filled with a train of bunches (white) and a single vertically displaced camshaft bunch on two turn closed orbits (green). In the bottom image, light from the main bunch train is collimated, leaving only light from a camshaft bunch.

The ALS has been successful in serving multiple users with a diverse set of requirements such as high-photon flux and brightness, a large range of wavelengths, variable polarization, and relatively short pulses. However, a major limitation of the ALS and other synchrotron light sources is the inability to serve two other classes of experiments simultaneously—brightness or flux-limited experiments and timing experiments. Typically, storage ring light sources operate with the maximum number of bunches possible, with a gap for ion clearing. By evenly distributing the beam current, the overall beam lifetime is maximized. The ALS normally operates with a train of 276 bunches out of a possible 328, with a single "camshaft" bunch in the middle of a 100-nanosecond gap.

The concept of using a camshaft bunch in multibunch operations started many years ago and originated out of the desire for some timing experimenters to operate during multibunch mode. However, most timing users cannot use the camshaft due to the short 100-ns gap. The ones that do must use gated detectors or expensive mechanical choppers to reduce the background from unwanted bunches. These choppers are challenging to fabricate and operate, and for beamlines that operate without a monochromator, they have to absorb about a kWof power while rotating at high speeds. Furthermore, the rotating frequencies of choppers constrain the repetition rate of the external laser.

The idea behind PSB operation is to use a high-repetition-rate (MHz), short-pulse (<100 ns) magnet to vertically kick the camshaft bunch relative to the bunch train. Then, by blocking the light from the multibunch train in the beamline, only light from the camshaft bunch reaches the experiment. Putting this bunch in the middle of the ion-clearing gap reduces the required bandwidth of the kicker magnets.

Recent ALS experiment in pseudo-single-bunch mode have demonstrated the feasibility of a "kick-and-cancel" scheme where, after two full turns around the storage ring, the orbit of the kicked bunch returns to its zero position in such a way that a second kick cancels the perturbation.

PSB operation has been tested at the ALS in various forms. For example, using one kicker magnet running at the ring repetition rate (1.5 MHz), PSB can be operated in a fixed high-repition-rate mode by permanently putting the camshaft bunch on a different orbit. It can also operate in an adjustable repetition-rate mode from milliseconds to microsecond with a novel ‘‘kick-and-cancel’’ (KAC) scheme. By adjusting the ring tune and the PSB kick pattern, the camshaft bunch can be first displaced to a different orbit and then kicked back to its original one within a few turns. This kick-and-cancel process can be repeated on demand, thus creating single-bunch pulses with adjustable repetition rates. This KAC scheme can significantly alleviate complications of using high-power choppers and substantially reduce the rate of sample damage. It allows the use of non-gated detectors, greatly increasing the variety and quality of experiments that can be done.

Research funding: This work is supported by the Director Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Operation of the ALS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences.